Method for crucible-free zone melting of semiconductor crystal rods

ABSTRACT

Polycrystalline semiconductor rods are converted to dislocation-free monocrystal rods by positioning a polycrystalline rod within a crucible-free zone melt environment with a seed crystal attached to one end thereof, generating a melt zone at the juncture of the seed crystal and the polycrystalline rod, controllably moving the melt zone away from the juncture and through the polycrystalline rod to a select point thereon, uniformly supporting the cone-shaped lower portion of the rod being processed so as to prevent oscillations and the like at the juncture of the rod in the seed crystal and controllably moving the melt zone from the select point to the remainder of the rod. A uniform support is preferably provided by an axially movable funnel-shaped casing which is attached to the seed crystal holding member and which, when moved into its operating position, is filled with a self-adjusting oscillation or vibration dampening means, such as molten metal, metal spheroids, quartz particles, sand, etc.

This is a division of application Ser. No. 525,641, filed Nov. 20, 1974,now U.S. Pat. No. 3,923,468.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to processing semiconductor crystal rods andsomewhat more particularly to a method and apparatus for crucible-freezone melt processing of a semiconductor crystal rod whereby formation ofdislocations and irregularities within the process crystal are avoided.

2. Prior Art

Semiconductor crystals, particularly those composed of silicon, aregenerally produced by a crucible-free zone melt process whereby amonocrystalline seed or nucleation crystal having a relatively smalldiameter is melt-connected, as with the aid of an induction heatingcoil, to an end of a relatively large diameter polycrystallinesemiconductor member and a melt zone is generated at the junction of theseed crystal and the polycrystalline member and passed one or more timesalong the length of the polycrystalline member. The melt zone is movedby providing relative movement between the polycrystalline member and aheat source, such as an induction heating coil, which may be the sameone used to melt-connect the seed crystal with the polycrystallinemember or be different therefrom. In this manner, a polycrystallinemember is purified of any foreign constituencies and simultaneouslyconverted into a monocrystalline member.

In the production of semiconductor components from so-producedsemiconductor materials, it is desirable that the semiconductor materialbe as free as possible from dislocations and other irregularities whichmateriall interfere with the electrical properties of the semiconductorcomponents produced therefrom. Further, the presence of dislocations,etc. within the semiconductor material decreases the life of minoritycarriers within such semiconductor materials.

German Auslegeschrift No. 1,079,593 (which generally corresponds toBritish Patent No. 889,160) suggests that dislocations in rod-shapedsemiconductor members may be decreased at the melt-connected juncture ofthe seed crystal and such semiconductor members by decreasing thecross-section of the semiconductor member at the direct proximity ofsuch melt-connected juncture prior to the last pass of the melt zonethrough the semiconductor member. Dislocations which may be present inthe seed crystal are thus given a chance to heal in the thus-producedthin connecting piece or bridge between the seed crystal and thesemiconductor member.

German Patent No. 1,128,413 (which generally corresponds to U.S. Pat.No. 3,175,891) discloses that substantially dislocation-free rod-shapedsilicon monocrystals may be produced, for example, by controlling therate or speed of travel of one or more passages of a crucible-free zonemelt through the rod. This reference suggests that all passages of themelt zone start in the seed crystal and that the travel speed of themelt zone in the seed crystal should be controlled so as to be in therange of about 7 to 15 mm/min. During the last pass of the melt zone,the silicon rod cross-section at the juncture of the seed crystal andthe silicon rod is constricted by a temporary relative movement of therod ends at a speed greater than 25 mm/min. while the speed of the meltzone is steadily decreased from this constriction point until the fullcross-section of the rod is attained. Thereafter, the melt zone is movedthrough the rod at a speed less than about 7 mm/min.

It has been noted that when monocrystalline semiconductor rods of afairly large diameter are being produced by the crucible-free zone meltprocess, the rod-shaped monocrystals which grow at the seed crystal atthe last passage of the melt zone tend to oscillate, particularly at thethin-connecting or bridging piece between the monocrystal and the seedcrystal. This drawback is particularly acute when thick monocrystallinesemiconductor rods are being produced, for example, by way of shorteningor compacting during a crucible-free zone melt process. Theseoscillations appear to cause the development of dislocations and otherirregularities in monocrystals of the material as it becomes rigidduring the last passage of the melt zone through the semiconductor rod.In addition, such oscillations often cause a dripping of molten materialfrom the melt zone or even a breakage of the thin-connecting or bridgingpiece between the seed crystal and the semiconductor rod, which, ofcourse, causes an interruption in the melt zone process.

I have disclosed in German Offenlegungsschrift No. 1,519,901 a means ofsupporting the ends of a crystal rod at the juncture thereof with a seedcrystal which comprises a finger-like support means that is positionedon the upper edge of a casing and which is axially movable and enclosesthe mounting for the seed crystal. However, this arrangement does notcompletely obviate vibrations or oscillations during the growth of verythick (i.e. having a diameter larger than 30 mm) dislocation-freesemiconductor monocrystalline rods since the finger-like supports do notuniformly touch the overall round cone portion of such rod. Due to thisinstability, increased oscillations may be produced opposing thesupporting effect desired or may even eliminate any beneficialsupporting action.

SUMMARY OF THE INVENTION

The invention provides an improved zone-melt process and apparatustherefor which substantially eliminates or prevents the development ofoscillations at the juncture of a seed crystal and a semiconductor rodduring the passage of, for example, a last pass of the melt zone throughthe semiconductor rod.

In accordance with the principles of the invention, a movablefunnel-shaped support means is coupled with a mounting for a seedcrystal so that upon activation of the support casing, it moves to anupper or support position and completely and uniformly encases the conearea of the rod. The support means may include a funnel-shaped casingmovable to the support position and a means for filling the casing witha vibration or oscillation dampening means which provides a uniformcontact with the cone area of the rod whereby all points about the conearea circumference are completely supported by such dampening means. Incertain embodiments of the invention, such dampening means may comprisegranular silicon, granular quartz or sand. In other embodiments of theinvention, such dampening means may comprise a liquified metal,preferably molten lead or indium, which becomes rigid within thefunnel-shaped casing to provide a desired support. In other embodimentsof the invention, the vibration or oscillation dampening means maycomprise metal spheroids or the like in an extremely dense distribution.In yet other embodiments of the invention, the dampening means maycomprise inserts of an extremely pure metal, for example, aluminum,attached to the upper periphery of the funnel-shaped casing whichcontact the cone area of the rod upon upward movement of the casing andmelt with the rod material to form an eutectic mixture between the metaland the rod material, for example, silicon, which then solidifies andprovides a solid connection between the casing and the rod. These lastembodiments, as compared to the suggestions of GermanOffenlegungsschrift No. 1,519,901, provide a uniform dampening andsupporting function even at any and all non-uniform points along thecone area circumference during the alloying process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional elevated view of a lower rodmounting arrangement for use in a crucible-free zone melt process whichschematically illustrates an embodiment of the invention in anon-operative position;

FIG. 2 is a somewhat similar view to that of FIG. 1 and schematicallyillustrates an embodiment of the invention in an operative position; and

FIG. 3 is a partial cross-sectional elevated view of a lower rodmounting arrangement for use in a crucible-free zone melt process, whichschematically illustrates another embodiment of the invention in anoperative position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides an improved method and apparatus for producingdislocation-free monocrystalline semiconductor rods.

In accordance with the principles of the method embodiments of theinvention, a polycrystalline semiconductor rod, such as primarilycomposed of silicon, is melt-connected to a seed crystal and positionedwithin a crucible-free zone melt environment. A melt zone is generatedat the juncture between the rod and the seed crystal and controllablymoved through the rod to a select point thereon, preferably at leastabout 10 cm above the cone-shaped area of the rod and then thecone-shaped lower area of the rod is uniformly supported so as toprevent any ocsillations and the like from forming at the juncture ofthe rod in the seed crystal. Thereafter, the melt zone is againcontrollably moved through the remainder of the rod. In this manner,oscillations at the juncture between the semiconductor rod and the seedcrystal are avoided so that dislocations and other irregularities cannotdevelop within the processed rod.

In accordance with the principles of the apparatus embodiments of theinvention, a funnel-shaped casing is appropriately coupled to a seedcrystal mounting means so as to be axially and rotatably movable withsaid mounting means and axially movable relative to such mounting means.At its uppermost position, the casing uniformly encloses the conicalarea of the crystalline rod being processed. Self-adjusting vibrationdampening means, such as molten metal, particulated quartz, sand,silicon, etc. is preferably provided within the casing to positivelyprovide anti-oscillation support and uniformly contact any irregularpoints about the rod circumference.

An extremely economical and reliable oscillation or vibration arrestingaction is obtained by the addition of a dampening means, such asgrain-like silicon, quartz, sand and/or metal particles. Such dampeningmeans, which may also include liquified or molten metal, is essentiallyself-adjusting to any irregularities in the rod circumference andprovides a uniform and complete support for the enclosed area thereofand prevents oscillations or the like so that dislocation-free crystalsare readily and reliably produced by following the principles of theinvention.

In one preferred embodiment of the invention, the dampening meanscomprises rigid inserts composed of a very pure metal having arelatively low melting point, such as aluminum, which are positioned atthe upper periphery of the funnel-shaped casing so that upon the upwardmovement of the casing, the inserts contact adjacent points of the rodand the ends of the inserts melt to form a eutectic mixture with the rodmaterial and then solidify to provide positive anti-oscillation support.In comparison with the arrangement suggested by GermanOffenlegungsschrift No. 1,519,901, the instant invention, including theabove preferred embodiments thereof, provides a uniform support for anyand all uneven points or the like at the rod circumference therebyavoiding the drawbacks noted earlier and providing positive and completesupport for the growing monocrystalline rods.

In accordance with the principles of the invention, the uniform supportfor the bottom of the rod being processed is provided when the melt zonehas traveled through the rod to a select point thereon, preferably atleast about 10 cm above the highest reach of the funnel-shaped supportmeans.

A preferred embodiment of an apparatus constructed and operable inaccordance with the principles of the invention comprises two verticallyopposing and approximately coaxially aligned rod mounting members in acrucible-free zone melt apparatus which may typically include aninduction heating coil and an enclosed chamber along with means forsupplying energy to the coil and for supplying a select atmosphere tothe chamber. At least one of the rod mounting members is provided with aseed crystal which is melt-connected to a rod to be treated (i.e. a meltzone is generated at adjacent surfaces of the seed crystal and the rodand the molten surfaces are brought together to form a unified moltenmass which is allowed to solidify and form a solid juncture. The rodmounting member having the seed crystal therein is also coupled with afunnel-shaped hollow casing which is axially and rotatably movable withthe mounting member and is independently axially movable relative to themounting member. This funnel-shaped casing is axially movable a distancesufficient to encompass the cone area of a rod above a seed crystal andmay be associated with a means for providing a self-adjusting vibrationor oscillation dampening means to the casing. The self-adjustingoscillation dampening means is selected from the group consisting ofparticulate silicone, quartz, sand, metal (metal spheroids), liquifiedand re-solidifiable metal having a relatively low melting point, i.e.below the melting point of the rod being treated, such as aluminum, leador indium, which may be provided in initially molten form to simply fillany space between the hollow funnel-shaped casing or be provided assolid inserts, the ends of which melt upon contact with the hot conearea of the rod being processed so as to form a eutectic mixture andupon solidification form a solid support for the rod. The flowabledampening means may be added to the funnel-shaped casing with the aid ofan appropriate filling device, such as a conduit and the non-flowabledampening means may be added to the funnel-shaped casing during assemblythereof.

In the drawings, only essential features of the crucible-free zone meltapparatus have been illustrated in order to simplify the understandingof the invention and workers in the art will readily recognize andsupply other elements and/or features required to obtain an operablezone melt apparatus and such operable apparatus will hereinafter and inthe claims be referred to as a crucible-free zone melt apparatus and/orenvironment.

As shown at FIG. 1, a monocrystalline seed 2 is mounted in a mountingmember 3 within a crucible-free zone melt apparatus and/or environmentZM. The seed crystal 2 is melt-connected with the lower end of asemiconductor crystalline rod 4 so as to form a juncture 2atherebetween. The rod 4 and the seed crystal 2 may, for example, becomposed of silicon. A melt zone 6 is generated, as with the aid ofinduction heating coil 5, and controllably moved in an axial directionthrough the rod 4 from the juncture 2a upward to a select point Pthereon. Movement of the melt zone is achieved by relative movementbetween the rod 4 and the heat coil 5.

A hollow funnel-shaped casing 7, for example, composed of titanium,steel, silicon or graphite, is mounted to encase the mounting member 3and since the drive means (schematically illustrated as the verticallydouble-headed arrow and the horizontally curved double-headed arrow) forthe mounting member is outside the bottom wall 11 of the crucible-freezone melt apparatus ZM, the casing 7 is axially and rotatably movablewith the mounting member 3. In addition, casing 7 is independentlyaxially movable relative to the mounting member 3. Thus, when themounting member 3 is axially or rotatably moved, the casing 7 islikewise moved but the casing 7 may be axially moved independent of themounting member 3. The funnel casing 7 is supported on a stud 8 which isattached to a pin 9 arranged within the mounting member 3 and which isaxially movable. The stud 8 may be positioned within a guide slot 10within the mounting member 3. Workers in the art will readily appreciatethat other coupling means may also be utilized in accordance with theprinciples of the invention. Reference numeral 11 designates a portionof the bottom wall of an enclosed chamber typically utilized in acrucible-free zone melt apparatus and various seal means, drive means,atmosphere control means, energy supply means, etc. have been omittedfrom the drawings in order to simplify the understanding of theinvention and to provide a better overall view of the essential featuresof the invention.

FIG. 1 illustrates the relative position of the elements before the rodend 4a is provided with support. The end 4a at this stage has grown as amonocrystal without dislocations at the relatively thin connecting orbridging piece 12 during a so-called bottle-neck-shaped constriction ornecking process whereby a melt connection was formed between the seedcrystal and the rod 4 and the melt zone moved therefrom through the rodone or more times. Since during such a process, both the rod 4 and theseed crystal 2 are rotated about their respective axes, there is adanger that the end 4a of the rod 4 which grows at thebottle-neck-shaped constriction 12 may begin to oscillate or vibratewhen the melt zone 6 moves sufficiently from the juncture 2a between theseed crystal 2 and the rod 4.

In instances where a silicon rod of about 40 mm diameter is being soprocessed, the distance between the bridging piece 12 and the melt zoneat which such oscillations are noted is about 70 cm. The amplitudes ofsuch oscillations are often so great that the growth process must beinterrupted and corrective measures taken.

In accordance with the principles of the invention, before the melt zone6 reaches the critical distance or point from the bottle-neck-shapedconstriction 12, the funnel-shaped casing 7 is moved upwardly with theaid of the pin 9 so as to encase the cone area 4a of the rod 4positioned above the seed crystal 2, such as shown in FIGS. 2 and 3. Inorder to insure a positive support and oscillation arrest, aself-adjusting dampening or stabilizing means 13 is provided between theinterior of the funnel-shaped casing 7 and the periphery of the lowerend 4a of the rod 4.

FIG. 2 illustrates a funnel-shaped casing 7 being filled with thedampening means 13, such as particulate quartz or sand, via a fillingconduit 14. Granular silicon, metal spheroids, sand or a compatible(i.e. substantially inert and non-contaminating) metal, such as moltenlead or indium, which solidifies within the casing 7 upon coming incontact with the relatively hot lower rod portion may also be used inplace of the particulate quartz or sand. As is apparent from FIG. 2,once the casing 7 has been moved to its operative or supportingposition, the rod 4 can no longer oscillate on the bottle-neck-shapedconstriction. The lower rod portion 4a follows the seed crystal and thenarrow constriction or bridging piece 12 is already sufficiently cold atthis time so that no dislocations or the like can form. The remainingreference numerals shown in FIG. 2 are identical to those utilized inFIG. 1 and refer to identical elements discussed in conjunction withFIG. 1.

FIG. 3 illustrates another embodiment of the invention for supporting agrowing monocrystal so as to avoid formation of dislocations or otherirregularities therein. In this embodiment, inserts 15 are attached viamountings 16 along the upper periphery of a funnel-shaped casing 7 sothat such inserts gently touch the periphery of the cone area 4a whenthe casing is moved to its upward position. The inserts 15 are composedof a compatible, relatively low melting, extremely pure metal, forexample, aluminum, which melts upon contact with the still-hot rodmaterial and alloys with or forms a eutectic mixture therewith. Whilethe melt zone continues its passage upwardly through the remainingportions of the rod 4, the alloyed area of the rod cone circumferencecools and a solid connection forms which prevents any oscillations orvibrations and provides a uniform support for any and all uneven pointson the cone circumference. In this embodiment, it is important to selecta metal for the inserts which has an appropriate melting point. Theremaining reference mumerals in this Figure are identical with thoseutilized in FIGS. 1 and 2 and refer to similar elements as discussed inconjunction with FIGS. 1 and 2.

As is apparent from the foregoing specification, the present inventionis susceptible of being embodied with various alterations andmodifications which may differ particularly from those that have beendescribed in the preceding specification and description. For thisreason, it is to be fully understood that all of the foregoing isintended to be merely illustrative and is not to be construed orinterpreted as being restrictive or otherwise limiting of the presentinvention, excepting as it is set forth and defined in thehereto-appendant claims.

I claim as my invention:
 1. A method of crucible-free zone meltprocessing of a semiconductor crystal rod comprising:arranging asemiconductor crystal rod within a crucible-free zone melt environmentwith a seed crystal attached to a lower end thereof; generating a meltzone at the juncture of said seed crystal and said rod and controllablymoving said melt zone away from said juncture through said rod to aselect point thereon; uniformly supporting said lower end of said rod byaxially upwardly moving a hollow funnel-shaped casing from a positionbelow said juncture between the seed crystal and the rod to a positionsurrounding the lower end of said rod, said casing having relatively lowmelting pure metal inserts attached at the upper periphery of saidfunnel-shaped casing, said inserts melting upon contact with thestill-hot lower end of the rod as such rod is traversed by said meltzone so as to form a eutectic mixture with the material of said rod andthen solidifying to form a solid connection between said casing and thelower end of said rod; and controllably moving said melt zone from saidselect point through the remaining portion of said rod.
 2. A method asdefined in claim 1 wherein said inserts are composed of aluminum.